Injection molding of part having nonuniform thickness

ABSTRACT

Injection molding of a part having a nonuniform thickness is provided. One disclosed embodiment of an injection molding device includes one or more side walls, a first mold surface intersecting the side walls and being stationary with respect to the side walls, and a second mold surface intersecting the side walls so as to define with the side walls and the first mold surface a cavity configured to receive a metered amount of injected molten thermoplastic material. The second mold surface is moveable toward the first mold surface in such a manner that a first end of the second mold surface moves a larger physical travel distance toward the first mold surface than does a second end of the second mold surface during a molding process.

BACKGROUND

Injection molding is a manufacturing process that may be used forproducing plastic parts. Injection molding generally involves injectingmolten plastic into a mold, and then allowing the plastic to cool andsolidify. However, many plastics shrink while cooling. Therefore, it maybe challenging to manufacture parts of a precise thickness via injectionmolding. Further, because the magnitude of shrinkage is dependent uponthe thickness of the plastic, various portions of a nonuniform plasticpart may shrink at different rates, thereby compounding the difficultiesin molding precision parts via injection molding.

SUMMARY

Various embodiments related to injection molding of a part having anonuniform thickness are disclosed herein. For example, one disclosedembodiment provides an injection molding device comprising one or moreside walls, a first mold surface intersecting the side walls and beingstationary with respect to the side walls, and a second mold surfaceintersecting the side walls so as to define with the side walls and thefirst mold surface a cavity configured to receive a quantity of injectedmolten thermoplastic material. Further, the second mold surface isconfigured to be moveable toward the first mold surface in such a mannerthat a first end of the second mold surface moves a larger physicaltravel distance toward the first mold surface than does a second end ofthe second mold surface when molding the thermoplastic material.

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter. Furthermore,the claimed subject matter is not limited to implementations that solveany or all disadvantages noted in any part of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exploded view of an embodiment of an example injectionmolding device according to the present disclosure.

FIG. 2 shows a schematic depiction of an embodiment of a part havingnonuniform thickness molded via the embodiment of FIG. 1.

FIG. 3 shows a flow diagram of an embodiment of a method of injectionmolding a part having a nonuniform thickness.

FIG. 4 shows a schematic depiction of a sequence of mold movements madeduring an embodiment of an injection molding process according to thepresent disclosure.

DETAILED DESCRIPTION

Injection molding of a part having a nonuniform thickness, such as anoptical wedge, may be challenging in that materials used for such partsmay shrink during solidification, thereby causing non-uniform shrinkagedue to the varying thickness of the part. Shrinkage during molding of auniform part may be compensated for by moving one surface of a moldtoward another surface in a linear manner as the plastic cools and sets.However, in the case of a nonuniform part, such linear motion may notwork, as thinner portions of the part that undergo less shrinkage maycause the mold to lock, and thereby not permitting further motion of themold as the thicker portion of the part continues to set up. This maycause the final part to have incorrect dimensions, and also may impactreproducibility of the molded part.

Accordingly, injection molding of a part having a nonuniform thicknessas described herein utilizes a mold surface configured to be movedtoward another mold surfaced in a nonlinear manner (e.g. by allowingrotation of the mold surface about an axis) when molding thethermoplastic material. This allows the mold to continue to move in thecase where a portion of the molded part spaced from the axis of moldrotation cures more slowly than a portion of the molded part locatedadjacent to the axis of mold rotation. Further, in some embodiments, themold surface may be both linearly and rotatably movable with respect toanother mold surface to allow different motion to occur during differentphases of a cooling process. While the disclosed embodiments utilize arotational mechanism to allow such nonlinear motion to occur, it will beunderstood that a mold according to the present disclosure may utilizeany other suitable mechanism for nonlinear motion.

FIG. 1 shows an exploded schematic view of an embodiment of an exampleinjection molding device 100 that may be used for molding a part havinga nonuniform thickness, such as example part 102, depicted schematicallyas an optical wedge and shown in perspective in FIG. 2. Returning toFIG. 1, injection molding device 100 may include a plurality of sidewalls, including side wall 104 and side wall 106, as well as side wallsthat intersect side walls 104 and 106 to form a rectangular structure.Injection molding device 100 further include major top and bottomsurfaces (referring to the orientation of FIG. 1), which are referred toherein as a first mold surface 108 and a second mold surface 110. Firstmold surface 108 may intersect the side walls such as side wall 104 andside wall 106, and may be stationary with respect to the side walls.Second mold surface 110 may also intersect the side walls, such as sidewall 104 and side wall 106, so as to define with the side walls andfirst mold surface 108 surface a cavity. Such a cavity may be configuredto receive a quantity of injected molten thermoplastic material, whichupon solidifying becomes part 102. As mentioned above, second moldsurface 110 may be moveable toward the first mold surface 108 in such amanner that a first end 112 of second mold surface 110 moves a largerphysical travel distance toward first mold surface 108 than does asecond end 114 of second mold surface 110 when molding the thermoplasticmaterial. Such a configuration will be described in more detail asfollows with reference to FIGS. 3 and 4.

FIG. 3 shows a flow diagram of an embodiment of a method 300 ofinjection molding a part having a nonuniform thickness. First, method300 may optionally include, at 302, heating a first mold surface of aninjection mold such that plastic injected into the mold does notinstantly start to harden on the first surface. Further, someembodiments of method 300 may optionally include, at 304, heating asecond mold surface of the injection mold, where the second mold surfaceis linearly and rotatably moveable toward the first mold surface. FIG.4, which shows a schematic depiction of a time sequence for anembodiment of injection molding of a part having a nonuniform thickness,shows examples of first and second mold surfaces respectively at 402 and404. A heater is shown for surface 404 schematically at 405. The heateris not shown for surface 402, or other mold surfaces, for the purpose ofclarity. It will be understood that other mold surfaces than surfaces402 and 404, such as the side walls 406 and 408, also may be heatedprior to injection of plastic into the mold.

Returning to FIG. 3, at 306 method 300 includes injecting a moltenthermoplastic material into the mold cavity. In some embodiments, themold cavity may be defined by a first mold surface, a second moldsurface, and a plurality of sides extending between the first moldsurface and the second mold surface, where the second mold surface beingmoveable toward the first mold surface. The thermoplastic material maybe injected into the cavity in any suitable manner. For example, in someembodiments, the molten thermoplastic material may be injected throughan opening in the second mold surface, while in other embodiments, themolten thermoplastic material may be injected into the mold at any othersuitable location.

Referring again to FIG. 4, at time t₁, mold cavity 410 has been filledwith thermoplastic material 412. Mold cavity 410 may be initiallydefined such that a thin end and a thick end of the molten plastic arethicker than desired for the component to accommodate the greater volumefor the molten thermoplastic material relative to a hardenedthermoplastic material. Then, as is described in more detail below,during cooling of the thermoplastic material, the second mold surface404 is moved toward the first mold surface 402 to keep the mold surfacesin contact with the thermoplastic material as the material cools,thereby achieving desired dimensional and surface properties in thefinal molded part.

The thermoplastic material may be any suitable material. In the case ofan optical wedge, the thermoplastic material may be transparent, and inmore specific embodiments, may comprise a material such as poly(methylmethacrylate) (PMMA), polycarbonate and/or polycyclic olefin. It will beunderstood that these materials are presented for the purpose ofexample, and that any other suitable material may be used in any othersuitable molding process.

As described above, one or more of the side walls, the first moldsurface and the second mold surface may be heated prior to and/or whileinjecting the molten thermoplastic material. Such heating of the sidewalls and cavity surfaces may prevent the thermoplastic material fromthermal shock upon injection.

Continuing with FIG. 3, at 308, method 300 includes, during cooling ofthe thermoplastic material in the mold cavity, moving the second surfacetoward the first mold surface such that a first end of the second moldsurface moves a larger physical travel distance than does a second endof the second mold surface, thereby forming the part having thenonuniform thickness. In some embodiments, as indicated at 310 and 312,respectively, 308 of method 300 may include linearly moving androtatably moving (i.e. rotating) the second mold surface toward thefirst mold surface, either simultaneously or during different phases ofa molding process. The term “linearly moving the second mold surfacetoward the first mold surface” refers to movement of the second moldsurface such that a first end of the second mold surface moves a samephysical travel distance as does a second end of the second moldsurface. As an example, at time t₂ FIG. 4 illustrates at 414 linearcompression being applied to the second mold surface 404 in a directiontoward first mold surface 402, such that a first end 416 of the secondmold surface 404 and a second end 418 of the second mold surface 404move approximately a same physical travel distance toward first moldsurface 402. In some embodiments, injection molding device 400 mayinclude a physical stop to limit an amount of physical travel distanceof the second mold surface so that second mold surface 404 may belinearly moved toward first mold surface 402 for a portion of its travelpath toward first mold surface 402. While the second mold surface 404that is movable toward the stationary first mold surface is depicted ascomprising an entire bottom side of the mold cavity (referring to theorientation of the mold shown in FIG. 4), it will be understood that insome embodiments, the second mold surface may comprise only a portion ofa side of the mold cavity.

Continuing with FIG. 4, after a predetermined distance of travel ofsecond mold surface 404, at time t₃ the second mold surface 404 isrotatably moved toward first mold surface 402 about an axis at thesecond end 418 of second mold surface 404. As such, first end 416 ofsecond mold surface 404 moves a larger physical travel distance thandoes the second end 418 of second mold surface 404.

Second mold surface 404 may be rotated in any suitable manner. Forexample, injection molding device 400 may include a hinge 420 aboutwhich second mold surface 404 may rotate. Such a hinge 420 may becoupled to one or more sidewalls such as side wall 408, and may allow atleast a portion of the second mold surface 404 to rotate about an axisat the second end 418 of second mold surface 404. It will be understoodthat the term “hinge” as used herein comprises hinges, pivots, and othersuch rotational couplings of two parts.

Thus, as shown in FIG. 4, linear movement of second mold surface 404toward first mold surface 402 as illustrated at t₁ may be used toachieve a desired thickness at a thin end of the part. Then, asillustrated at t₂, second mold surface 404 may then be rotatably movedtoward first mold surface 402 to achieve a desired thickness at thethick end of the part. Upon doing so, the final part has the desirednonuniform thickness. Thus, although plastics may shrink by a volumefraction while cooling, injection molding as described herein allows forprecisely controlling of the various thickness of a part by linearly androtatably moving the second mold surface toward the first mold surfacein a controlled manner so that the mold provides compressive force tothe entire part during its entire cooling cycle.

An injection molding machine may be configured to move second moldsurface 404 toward first mold surface 402 in any suitable manner. Forexample, in some embodiments, a molding machine to which the injectionmolding device 400 is coupled may include a compression core configuredto move the second mold surface toward the first mold surface. Such acompression core may include a cam mechanism, hydraulic cylinders, gearrack and pinion, or any other suitable mechanism for moving the secondmold surface.

As mentioned above, injection molding as described herein may be used tocreate an optical wedge that may be used as a light-guide in a largedisplay, or may be used in a smaller form factor suitable for aninteractive keyboard, mouse or webcam, or in any other suitable device.Such an optical wedge may also be used as a backlight for a liquidcrystal display (LCD). Some optical wedges may comprise a Fresnel lensformed in one side of the wedge to focus and redirect light entering thewedge from a light/air interface of the wedge. Therefore, in this case,one of the side walls of the injection molding device may comprise aFresnel lens molding surface configured to form such a Fresnel lens inthe optical wedge. Further, in order to achieve a desired smoothness forthe surface of a precision molded object, such as a light guide, one ormore of the side walls and cavity surfaces of the injection moldingdevice may be diamond-lapped, or may be formed from another processsuitable for making highly smooth surfaces.

While disclosed herein in the context of an optical wedge, it should beunderstood that the embodiments herein are illustrative and notrestrictive, since the scope of the invention is defined by the appendedclaims rather than by the description preceding them, and all changesthat fall within metes and bounds of the claims, or equivalence of suchmetes and bounds thereof are therefore intended to be embraced by theclaims.

1. An injection molding device for molding a part having a nonuniformthickness, the injection molding device comprising: one or more sidewalls; a first mold surface intersecting the one or more side walls andbeing stationary with respect to the one or more side walls; and asecond mold surface intersecting the one or more side walls so as todefine with the one or more side walls and the first mold surface, acavity configured to receive a metered amount of injected moltenthermoplastic material, the second mold surface being moveable towardthe first mold surface in such a manner that a first end of the secondmold surface moves a larger physical travel distance toward the firstmold surface than does a second end of the second mold surface whenmolding the thermoplastic material.
 2. The injection molding device ofclaim 1, further comprising a physical stop configured to stop movementof the second mold surface toward the first mold surface.
 3. Theinjection molding device of claim 1, wherein the second mold surface isrotatably moveable toward the first mold surface about an axis locatedat the second end of the second mold surface.
 4. The injection moldingdevice of claim 1, wherein the second mold surface is configured to belinearly moved toward the first mold surface for a portion of a travelpath relative to the first mold surface.
 5. The injection molding deviceof claim 4, further comprising a hinge coupling a portion of the secondmold surface to one of the side walls, the hinge allowing the portion ofthe second mold surface to rotate about an axis at a second end of theportion of the second mold surface.
 6. The injection molding device ofclaim 1, further comprising one or more heaters for heating the one ormore side walls, the first mold surface, and/or the second mold surface.7. The injection molding device of claim 1, coupled to a moldingmachine, the molding machine including a compression core configured tomove the second mold surface toward the first mold surface.
 8. Theinjection molding device of claim 7, wherein the injection moldingdevice is configured to form an optical wedge, and wherein a side wallof the injection molding device comprises a Fresnel lens moldingsurface.
 9. A method of injection molding a part having a nonuniformthickness, the method comprising: injecting a molten thermoplasticmaterial into a mold cavity, the mold cavity being defined by a firstmold surface, a second mold surface, and a plurality of sides extendingbetween the first mold surface and the second mold surface, the secondmold surface being moveable toward the first mold surface; and duringcooling of the thermoplastic material in the mold cavity, moving thesecond mold surface toward the first mold surface such that a first endof the second mold surface moves a larger physical travel distance thandoes a second end of the second mold surface, thereby forming the parthaving the nonuniform thickness.
 10. The method of claim 9, wherein thethermoplastic material is optically transparent.
 11. The method of claim10, wherein the thermoplastic material comprises poly(methylmethacrylate).
 12. The method of claim 10, wherein the thermoplasticmaterial comprises polycarbonate.
 13. The method of claim 10, whereinthe thermoplastic material comprises polycyclic olefin.
 14. The methodof claim 9, further comprising heating one or more of the sides, thefirst mold surface, and the second mold surface during a moldingprocess.
 15. The method of claim 14, wherein the one or more of thesides, the first mold surface, and the second mold surface are heatedprior to injecting the molten thermoplastic material.
 16. The method ofclaim 14, wherein the one or more of the sides, the first mold surface,and the second mold surface are heated while injecting the moltenthermoplastic material.
 17. The method of claim 9, wherein moving thesecond mold surface includes linearly moving the second mold surfacetoward the first mold surface, and after a predetermined distance oftravel of the second mold surface, rotating the second mold surfacetoward the first mold surface about an axis at the second end of thesecond mold surface.
 18. The method of claim 9, wherein the part havingthe nonuniform thickness is an optical wedge and wherein the methodfurther comprises molding a Fresnel structure into an end of the opticalwedge.
 19. A method of injection molding of a part having a nonuniformthickness, the method comprising: heating a first mold surface of aninjection mold; heating a second mold surface of the injection mold, thesecond mold surface being linearly and rotatably moveable toward thefirst mold surface; injecting a metered amount of a moltenoptically-transparent thermoplastic material through an opening in thesecond mold surface; during cooling of the optically-transparentthermoplastic material, moving the second mold surface linearly towardthe first mold surface such that a first end of the second mold surfacemoves a same physical travel distance as does a second end of the secondmold surface; and after a predetermined distance of travel of the secondmold surface, rotating the second mold surface toward the first moldsurface about an axis at the second end of the second mold surface suchthat the first end of the second mold surface moves a larger physicaltravel distance than does the second end of the second mold surface. 20.The method of claim 19, wherein rotating the second mold surface towardthe first mold surface comprises rotating the second mold surface abouta hinge at the second end of the second mold surface.